The present invention relates generally to optical sensing systems, such as those which are used in hardcopy devices for scanning and/or printing images on print media, for example, using inkjet printing technology.
Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are described and shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, the Hewlett-Packard Company of Palo Alto, Calif. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a xe2x80x9cservice stationxe2x80x9d mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as xe2x80x9cspitting.xe2x80x9d The waste ink is collected at a spitting reservoir portion of the service station, known as a xe2x80x9cspittoon.xe2x80x9d After spitting, uncapping, or occasionally during printing, most service stations have a flexible wiper, or a more rigid spring-loaded wiper, that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
Optical sensors have been incorporated into various inkjet printing mechanisms, such as printers and plotters, for the past several years. These optical sensors illuminated the media using one to twelve light emitting diodes (xe2x80x9cLEDsxe2x80x9d). In U.S. Pat. No. 6,036,298, currently assigned to the present assignee, the Hewlett-Packard Company, a single monochromatic, or xe2x80x9cquasimonochromaticxe2x80x9d LED was proposed using a blue LED. This patent also has a detailed description of several prior art optical sensors, including those using the red and green LEDs. A single LED optical sensor emitting a blue-violet light was first introduced in the DeskJet(copyright) 990C model color inkjet printer last year. The single blue-violet LED illuminated the media, while two sensors received light reflected from the media, with one receiving diffuse light beams, and the other receiving specular light beams. Incoming light was restricted by two different stops, two rectangular windows having longitudinal axes which were perpendicular to one another. From information gathered by the sensor, the printer controller determined which type of media was entering the printzone and then adjusted the printing routines to provide an optimal image on the particular media used.
Unfortunately, all of these earlier optical sensors employed in inkjet printing mechanisms used bulky, commercial LEDs, which caused the sensors to occupy a large amount of space within the printing mechanism. It is believed that earlier this year, plotter designers for the Hewlett-Packard Company introduced a three LED optical sensor, using LEDs of the colors blue, green, and amber in the Designet(copyright) 10 ps, 20 ps and 50 ps models of color inkjet plotters. While the amount of space consumed by a sensor in a large floor mounted plotter has little impact on the overall desirability of the unit, in the desktop printing market, many consumers prefer a compact printing unit which occupies very little desk space, known in the art as having a small xe2x80x9cfootprint.xe2x80x9d Thus, in the desktop printer market, use of a wide bulky sensor mounted on the printhead scanning carriage increased the overall width of the printer by up to an inch (2.54 cm). While plotter designers were able to use optical sensors having multiple LEDs without impacting the overall plotter design, designers of desktop printers strived to find ways to use a single LED, for instance as described above in U.S. Pat. No. 6,036,298 and as sold in the DeskJet(copyright) 990C model color inkjet printer, mentioned above. Use of two or more LEDs in the desktop printer market was unthinkable, due to the adverse impact such a multiple LED sensor would have on a printer""s footprint, theoretically making a printer up to two inches (5.08 cm) wider. Such an additional width in a desktop printer could well make consumers turn away from the printer, and buy a more compact printer produced by a competitor, even at the expense of sacrificing the print quality benefits achieved by printers employing an optical sensor system. Furthermore, while these earlier optical sensor systems may have had some calibration at the factory, none are known to have had any way of automatically calibrating the sensors after the printing units left the factory.
One hand held color scanner has been developed by Color Savvy, of Springboro, Ohio, as described in the paper entitled xe2x80x9cAn LED Based Spectrophotometric Instrument, xe2x80x9d by Michael J. Vrhel, published as a part of the ISandT/SPIE Conference on Color Imaging: Device-Independent Color, Color Hardcopy , and Graphic Arts IV, San Jose, Calif., January 1999 (SPIE Vol. 3648, No. 0277-786X/98), as well as the system described in Color Savvy""s International Patent Application No. PCT/US97/16009, published Mar. 19, 1998, International Application No. WO 98/11410. Indeed, Color Savvy even advertises a scanning adapter that may be attached to the printhead scanning carriage of some inkjet printers, allowing the system to scan previously printed images. These devices made by Color Savvy are designed to xe2x80x9cseexe2x80x9d an infinite variety of different colors, shades and hues, and to accomplish this objective in a satisfactory manner, Color Savvy needs eight to sixteen different colored LEDs to illuminate the image. As mentioned above, such a bulky sensor having multiple LEDs will be too cumbersome for use in typical inkjet printers. Note that the Color Savvy adapter, when placed in an inkjet printer, rendered the unit unusable for printing.